In the UMTS (Universal Mobile Telecommunications System) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower latency and so on (see non-patent literature 1). Also, the specifications of LTE-A (also referred to as “LTE-advanced,” “LTE Rel. 10,” “LTE Rel. 11” or “LTE Rel. 12”) have been drafted for further broadbandization and increased speed beyond LTE (also referred to as “LTE Rel. 8” or “LTE Rel. 9”), and successor systems of LTE (also referred to as, for example, “FRA (Future Radio Access),” “5G (5th generation mobile communication system),” “NR (New Radio),” “NX (New radio access),” “FX (Future generation radio access),” “LTE Rel. 13,” “LTE Rel. 14,” “LTE Rel. 15” and/or later versions) are under study.
In LTE Rel. 10/11, carrier aggregation (CA) to integrate multiple component carriers (CC) is introduced in order to achieve broadbandization. Each CC is configured with the system bandwidth of LTE Rel. 8 as one unit. In addition, in CA, multiple CCs under the same radio base station (eNB (eNodeB)) are configured in a user terminal (UE (User Equipment)).
Meanwhile, in LTE Rel. 12, dual connectivity (DC), in which multiple cell groups (CGs) formed by different radio base stations are configured in a UE, is also introduced. Each cell group is comprised of at least one cell (CC). In DC, since multiple CCs of different radio base stations are integrated, DC is also referred to as “inter-eNB CA.”
Also, in LTE Rel. 8 to 12, frequency division duplex (FDD), in which downlink (DL) transmission and uplink (UL) transmission are made in different frequency bands, and time division duplex (TDD), in which DL transmission and UL transmission are switched over time and made in the same frequency band, are introduced.